This invention relates to a ceramic circuit board comprising a ceramic substrate having a glaze film formed thereon and a process for producing such a ceramic circuit board. The glaze film is subsequently overlaid with a functional film such as a ferromagnetic or conductive film.
A ceramic circuit board having a photolithographically patterned film of a ferromagnetic material, which serves as a magnetoresistive element having a resistivity which varies with magnetic field strength, has found applications as a magnetic sensor or MR head (magneto-resistive head), which is a kind of thin film magnetic head. Such a magnetic sensor is useful for counting revolutions of a part, for example. An MR head is a read only head, but it has a high reproducing output even at a low circumferential velocity since it detects the magnitude of magnetic flux, and it can be used to read a magnetic recording medium having a high recording density.
In a ceramic circuit board having a photolithographically patterned ferromagnetic film on a ceramic substrate, a glaze film having a smooth surface is normally interposed between the ceramic substrate and the ferromagnetic film in order to prevent the ferromagnetic film, particularly where the patterned film has areas in the form of thin lines, from breaking (disconnecting) due to projections or pores which inevitably exist on the surface of the ceramic substrate. The glaze film also minimizes fluctuations in the resistivity of the ferromagnetic film. Similarly, in a ceramic circuit board having, on a ceramic substrate, a conductive film of a metallic material which serves as a heating element, a glaze film is interposed between the ceramic substrate and the conductive film.
In the manufacture of these ceramic circuit boards, a glaze film is first formed on the surface of a ceramic substrate to produce a ceramic circuit board having a glaze film (such a board being hereinafter referred to as "glazed ceramic circuit board"). A functional film such as a ferromagnetic or conductive film is then formed on the glaze film of the glazed ceramic circuit board.
As schematically shown in FIG. 7, a conventional glazed ceramic circuit board has a ceramic substrate 11 usually made of alumina, on which a glaze film 13 and a conductor circuit 15 are formed. Such a glazed ceramic circuit board has conventionally been produced by firing a ceramic green sheet to form a ceramic substrate 11, applying a glaze-forming glass material and a conductive paste onto the substrate by screen printing, and firing again to form a glaze film 13 and a conductor circuit 15 on the substrate.
The ceramic circuit board can be used, for example, as a magnetic sensor, after a ferromagnetic material is deposited by sputtering on the glaze film 13 and patterned photolithographically so as to form a ferromagnetic film 17 serving as a magnetic sensor and a connecting circuit 19 is then formed so as to extend from the ferromagnetic film 17 to the conductor circuit 15 and transmit signals generated by the ferromagnetic film to the conductor circuit, as shown in FIG. 9.
However, the above-mentioned conventional glazed ceramic circuit board has the following drawbacks.
First, the glaze film 13 has a coefficient of thermal expansion which is significantly greater than that of the ceramic substrate 11. Therefore, the glaze film undergoes a greater contraction than the substrate does during cooling, which follows firing to form the glaze film and conductor circuit, thereby producing a thermal stress and warpage in the board, as shown in FIG. 8. When a thermal shock is applied to the thermally stressed ceramic circuit board during subsequent processing or in use, cracks 21 may form in the glaze film 13, resulting in disconnection of the ferromagnetic film 17 formed thereon.
Second, the ferromagnetic film 17 formed on the glaze film must have a uniform thickness, since the sensing accuracy of a magnetic sensor is significantly deteriorated if the thickness of the ferromagnetic film varies remarkably. In order to form a ferromagnetic film having a uniform thickness on a glaze film, the glaze film must have a smooth surface with a surface roughness of 0.05 .mu.m Ra or less, which is obtainable only when the glaze film has a sufficient thickness. For this reason, the glaze film 13 is usually formed so as to have a thickness of at least 30 .mu.m, which is substantially greater than the thickness of the conductor circuit 15 which is normally between about 5 .mu.m and about 15 .mu.m.
Therefore, there is a difference in level of 30 .mu.m or greater between the glaze film 13 and the ceramic substrate 11. As schematically shown in FIG. 10, such a big difference in level causes a connecting circuit 19, which is formed to connect the ferromagnetic film 17 formed on the glaze film 13 to the conductor circuit 15 which is, in turn, connected to external connecting pins 23, to become very thin and at times interrupted or disconnected on the peripheral wall 13A of the glaze film. This significantly reduces the reliability of connection by the connecting circuit 19. It is possible to alleviate this problem by forming the glaze film so as to have a gently declining peripheral wall rather than the depicted vertical one. Such a glaze film, however, will occupy an increased area on the ceramic substrate and hence hinders the reduction in size of ceramic circuit boards, which is always a goal in the art.